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Does low carb intake during easy runs promote more fat oxidization and improved performance?


The Impact of Low-Carbohydrate Diets on Endurance Performance: Insights from Research


Endurance athletes constantly seek dietary strategies to enhance performance, optimize energy utilization, and sustain effort over long durations. Among these, the low-carbohydrate, high-fat (LCHF) dietary approach has emerged as a contentious topic. Advocates praise its ability to enhance fat oxidation, while critics highlight its potential downsides, particularly in high-intensity scenarios. This blog examines the nuanced effects of low-carbohydrate diets on endurance performance from an evidence based perspective.





What Are Low-Carbohydrate Diets?


LCHF (low carbohydrate-high fat) diets typically consist of less than 100 grams of carbohydrates daily, with fat providing 75–80% of total energy intake. This macronutrient profile contrasts starkly with traditional high-carbohydrate (HCHO) diets commonly used by endurance athletes. By depleting carbohydrate reserves, LCHF diets shift the body’s reliance toward fat oxidation, theoretically tapping into a more abundant energy source while providing a potent stimulus for enhancing fat-burning metabolism. However, performance in endurance sports isn't just about energy availability—it's also about efficiency. This raises a crucial question: Should endurance athletes incorporate a low-carbohydrate diet? Let's examine what the evidence says.


Theoretical Basis for LCHF Diets in Endurance Sports


Endurance exercise demands sustained energy. Muscle glycogen in the form of stored carbohydrates, is the preferred fuel for high-intensity efforts, while fat oxidation dominates during prolonged, moderate-intensity activities. Advocates of LCHF diets argue that enhancing fat oxidation through carbohydrate restriction provides athletes with a competitive edge during long-duration events by preserving glycogen stores for critical race phases.

This approach aligns with the "train low, compete high" strategy, where athletes train under low-carbohydrate conditions to stimulate metabolic adaptations, such as increased mitochondrial enzyme activity, fat oxidation, and skeletal muscle oxidative capacity. However, the translation of these adaptations into performance improvements remains debated.





What Do the Studies Say?


1. Meta-Analysis: The Big Picture


A meta-analysis by Burke et al. (2020) (1) systematically evaluated the effects of ketogenic diets on endurance performance. While LCHF diets rapidly increased fat oxidation rates—up to 200% above baseline—they simultaneously elevated the oxygen cost of exercise by 5–8%. This increase in oxygen cost reflects reduced exercise economy, a critical determinant that can impair performance in high-intensity events.


Notably, despite glycogen restoration efforts, athletes following an LCHF diet demonstrated impaired carbohydrate oxidation (only 61-78% of baseline values) and a 2.2% performance decrease in 10,000-meter races. Conversely, athletes consuming high-CHO diets improved performance by 5.7% over the same period.


Key takeaway: Enhanced fat oxidation does not necessarily translate into better performance, particularly in high-intensity endurance events.


2. Hansen et al. (2005): The Role of Glycogen Depletion


One of the earliest studies investigating carbohydrate restriction's effects on skeletal muscle adaptation, Hansen et al., (2) demonstrated that training with low muscle glycogen led to greater increases in mitochondrial enzymes and time-to-exhaustion during exercise. However, this study was conducted on untrained participants, limiting its applicability to elite endurance athletes. Moreover, while cellular adaptations were evident, real-world performance outcomes were not assessed.


Key takeaway: Training with low glycogen may enhance metabolic efficiency but requires further research to determine its effect on competitive performance.


3. Marquet et al. (2016): The "Sleep Low" Strategy


Marquet et al. (3) introduced the "sleep low" protocol, wherein athletes performed evening training sessions to deplete glycogen stores, fasted overnight, and trained in a low-glycogen state the following morning. This approach improved cycling efficiency and 10 km running performance. However, it is worth noting that this strategy involves periodic carbohydrate restriction, not the sustained restriction characteristic of LCHF diets.


Key takeaway: Strategic, short-term carbohydrate restriction can improve endurance performance, but prolonged restriction may not yield the same results.


4. Meta-Analysis by Impey et al. (2018): A Mixed Verdict


A broader review by Impey et al. (4) analyzed various "train low" strategies, including fasted training and twice-daily sessions. Athletes periodically train with low CHO availability (e.g., 30–50% of training sessions) to enhance mitochondrial biogenesis and oxidative enzyme activity while competing with high CHO stores for optimal performance. The outcomes were as follows:


  • Performance Improvements: Only 37% of studies reviewed reported improvements in exercise performance with train-low strategies.


  • No Change: 63% of studies showed no significant impact on performance metrics like time-to-exhaustion (TTE) or time trials (TT).


Key takeaway: Train-low strategies show potential but require careful individualization and may not be universally effective.


5. The Impact of Ketogenic Low-Carbohydrate, High-Fat Diets on Endurance Athletes: A Systematic Review and Meta-Analysis


A recent study published in Nutrients (2021) (5) explored the effects of low-carbohydrate availability training strategies on endurance performance and physiological adaptations in athletes. The key findings provide deeper insights into the efficacy of these approaches:


  • The meta-analysis included 10 eligible studies. Examining the outcomes of aerobic capacity, exercise performance, and substrate oxidation, they found a significant effect only on respiratory exchange ratio (RER). No significant effects were observed on VO2max, maximum heart rate, time to exhaustion, or rating of perceived exertion. This finding aligns with previous studies indicating that LCHF diets have minimal impact on maximal aerobic capacity.





Why Doesn’t Fat Oxidation Translate into Better Performance?


  1. Increased Oxygen Cost: Fat oxidation requires more oxygen to produce the same amount of energy as carbohydrate oxidation. For high-intensity efforts, this inefficiency becomes a limiting factor, as oxygen delivery is already maxed out.


  2. Blunted Glycogen Utilization: Even after carbohydrate reintroduction, athletes adapted to LCHF diets exhibit impaired carbohydrate use. This blunted glycogen utilization compromises their ability to sustain high-intensity efforts.


  3. Reduced Exercise Economy: The increased reliance on fat as a substrate raises the metabolic cost of exercise, reducing overall efficiency.


What are the risks of low-carbohydrate training?


  1. Reduced training intensity: Athletes may struggle to maintain high training loads in low CHO states, potentially diminishing overall training quality.


  2. Immune suppression: Low CHO availability can increase susceptibility to illness, especially during prolonged or high-intensity sessions.


  3. Muscle protein breakdown: Without sufficient CHO, the body may break down muscle protein for energy, risking muscle mass loss.


  4. Blunted carbohydrate utilization: Chronic low CHO training may impair the ability to oxidize exogenous carbohydrates during competition, potentially impacting performance.


  5. Misapplication: Misinterpreting "train low" as "train zero" (completely avoiding carbohydrates) can lead to poor recovery, reduced performance, and energy deficits.


Mitigation strategies:


  • Balance low CHO sessions with high CHO sessions ("train smart").


  • Incorporate protein intake to mitigate muscle breakdown.


  • Use strategies like caffeine or CHO mouth rinses to maintain intensity during low CHO sessions.


Practical Implications for Endurance Athletes


Given the evidence, endurance athletes should approach LCHF diets with caution and is typically not recommended by myself. Here are some key considerations:


When to Use LCHF Diets


  • Ultra-Endurance Events: For events lasting several hours at moderate intensities, the enhanced fat oxidation from an LCHF diet may provide a competitive advantage.


  • Base Training: If you do plan on incorporating some carbohydrate restriction during training, it should be done sparingly (1-2 sessions/week) during base training which may improve metabolic flexibility and fat utilization.


When to Avoid LCHF Diets


  • High-Intensity Events: For races that involve sustained high-intensity efforts, such as marathons, triathlons, or time trials, the increased oxygen cost and reduced carbohydrate utilization associated with LCHF diets are likely to impair performance.


  • Competition Phase: Athletes should prioritize glycogen availability during competition phases to maximize performance.




The Bottom Line: Is LCHF Worth It?


The evidence suggests that while LCHF diets induce profound metabolic adaptations, these changes do not consistently translate into improved endurance performance, with most studies showing no significant enhancements to endurance performance. Meta-analyses and controlled studies highlight their benefits for fat oxidation but underscore their limitations for high-intensity efforts. For most endurance athletes, a traditional high-carbohydrate diet is superior for most distance events. I recommend this approach over a low-carbohydrate strategy.



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References

  1. Burke, L. M., et al. (2020). "Adaptation to a Low-Carbohydrate, High-Fat Diet is Rapid but Impairs Endurance Exercise Metabolism and Performance." Journal of Physiology. DOI: 10.1113/JP280221

  2. Hansen, A. K., et al. (2005). "Training with Low Glycogen Enhances Skeletal Muscle Adaptations." Journal of Applied Physiology. DOI: 10.1152/japplphysiol.00196.2005

  3. Marquet, L. A., et al. (2016). "Enhanced Endurance Performance by Periodized Carbohydrate Restriction." International Journal of Sports Nutrition and Exercise Metabolism. DOI: 10.1123/ijsnem.2016-0008

  4. Impey, S. G., et al. (2018). "Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization." Sports Medicine. DOI: 10.1007/s40279-018-0867-7

  5. Cao, J., Lei, S., Wang, X., & Cheng, S. (2021). The effect of a ketogenic low-carbohydrate, high-fat diet on aerobic capacity and exercise performance in endurance athletes: a systematic review and meta-analysis. Nutrients13(8), 2896. https://doi.org/10.3390/nu13082896

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